Patent Literature 1 discloses an optical system that uses a plane-symmetric imaging element to form an image of an object at a plane-symmetric position on top-surface side with respect to the element. The object is disposed on bottom-surface side of the element. A substrate of the plane-symmetric imaging element used in the optical system is provided with a plurality of through holes arranged in a matrix, and an optical element including two mirrored surfaces that are orthogonal to each other is formed on an interior wall of each of the holes. When light outputted from the object passes through the holes, the light is reflected once by each of the two mirrored surfaces, and an image of the reflected light is formed at a plane-symmetric position with respect to the element. This causes an observer to see a formed image (a real image) as if the image is floating above the top surface of the element.